Method and device for activating a physical and/or a chemical reaction in a fluid medium

ABSTRACT

It is possible to activate a physical reaction and/or a chemical reaction in a medium, comprising a solution and solid matter that is freely dispersed in said solution, by placing the mixture in a reactor that has two walls which are located opposite close to each other, whereby the mixture fills the space in between the two walls and forms therein a thin long layer in a direction that is defined by a geometrical axis parallel to the walls, by activating an agitating means that are disposed outside the reactor and capable of acting through said walls on an area of agitation covering part of the layer and having a small dimension in the direction of the geometrical axis, and by simultaneously displacing said agitating means in such a way that the area of agitation substantially spans the entire area located between the two walls.

BACKGROUND OF THE INVENTION

The invention relates to a method and a device for activating a physicalor chemical reaction, enabling the kinetics of the reaction to beincreased. It relates in particular to a process enabling precipitationof metals diluted in solutions, which consists in:

disposing the mixture in a reactor having two walls facing one anotherand close to one another, the mixture filling the space between the twowalls and forming therein a layer of small thickness and of great lengthin a direction defined by a geometric axis parallel to the walls,

agitating the solution by activation of an agitating means arrangedoutside the reactor to act through said walls on an agitation zonecovering a part of said layer and having a small dimension in thedirection of the geometric axis,

and moving the agitating means so that the agitation zone spansappreciably the whole of said space located between the two walls.

STATE OF THE ART

The document EP-A-0,014,109 describes a process and a device enablingphysical and/or chemical reactions to be fostered in a fluid medium bysubjecting a magnetic substance, dispersed in the fluid and playing aphysical and/or chemical role in the reaction to be fostered, to avariable magnetic field. The magnetic field is created by means ofdifferent electromagnetic coils arranged outside a recipient or reactorcontaining the fluid medium and the magnetic substance. The reactor is arevolution cylinder. The coils are preferably arranged on several levelsof the reactor in the heightwise direction so that the application zoneof the magnetic field covers a large portion of the reactor, withseveral electromagnets per level.

For certain of the physical and/or chemical reactions to be fostered bythe process and device described above, the maximum linear velocity ofthe fluid in the reactor proves determinant for the efficiency of thereaction and has to remain fairly low. This is the case in particularfor cementation reactions. What is referred to here as cementation isthe process consisting in replacing a relatively noble metal M_(N)present in a solution in ionic form by a more reactive metal M_(R)introduced in solid form, according to a precipitation reaction of thetype:

In a reaction of this type, the kinetics of the process are a functionof the surface offered by the solid reactive metal and of the noblemetal concentration of the solution. It is therefore preferable toensure rapid renewal of the solution in contact with the reactive metalso that the solution in the vicinity of the reactive metal is notdepleted in noble metal ions. It is at the same time preferable toincrease the reaction surface. However, if the size of the reactivemetal particles is decreased too much in order to increase theirreaction surface, it becomes difficult to ensure a sufficient relativeflow velocity of the solution with respect to the reactive metalparticles to prevent depletion of the solution mentioned above.Moreover, too high a flow velocity does not enable the solution to betreated in a single run, which means that the solution has to be passedseveral times over the same reactive metal bed, mixing it each time withnon-treated solution. To achieve optimal kinematics and globalefficiency, a compromise therefore has to be found between the size ofthe reactive metal particles and the relative velocity of the solutionwith respect to these particles. It is also necessary to preventprecipitation of the noble metal taking place at the surface of thereactive metal, for in this case the reaction would be quicklypassivated.

When the activation process by electromagnetic fields described above isimplemented within the scope of cementation reactions aiming to extracta noble metal such as copper, by means of iron used as reactive metal,application of an alternating magnetic field enables agitation of thesolution and speeding-up of its kinetics to be achieved. However, themean linear velocity of the solution in the active part of the reactorsubjected to the magnetic field must, for the reasons explained above,remain within a range whose upper bound is low. To give a precise idea,if three levels of four pairs of electromagnets are used, as describedin the document EP-A-0,014,109, with a solution containing 3 g/l ofcopper, the mean linear velocity of the solution is about 12 cm/s only.

Given this constraint, it is the cross-section of the active part of thereactor which determines the reactor flow rate. In a device of this kindhowever, the reactor cross-section is greatly limited by the power ofthe available electromagnets. In practice, the diameter used does notexceed 16 cm, whence a maximum flow rate not exceeding 10 m³/hr. Theseperformances are far from those expected industrially for metallurgicalprocesses if we consider that for an industrial installation enablingfor example 5,000 tons of copper to be produced per annum from asolution containing 3 g/l of copper, a flow rate of 190 m³/hr isnecessary, requiring with the technology described 20 reactors totalling240 pairs of electromagnets. The high costs arising from theelectromagnets should be underlined, which disqualify this type oftechnology. The electromagnets do in fact constitute an expensive itemin the investment budget. Furthermore they have a high operating cost asthey give rise to large energy expenses, not forgetting servicing andmaintenance costs.

The exchange surface between the reactive metal and the solution hasmoreover been attempted to be improved by means of fluidised beds. Anexample of implementation of these fluidised beds is described in thePatent U.S. Pat. No. 3,154,411. In this embodiment, nearly 99% of thecopper dissolved in a solution is extracted. However, the iron usedreacts greatly with the acidity of the medium with the consequence of alarge amount of hydrogen being given off and a reduced iron yield.Moreover, this process is not continuous and the copper cements are richin iron. Furthermore, Swiss Patent No. 9827/72 discloses that thedifficulties proper to fluidised beds can be overcome by performingcementation of metals such as Cu, Cd, Co, etc. on zinc granulesfluidised in a mechanically agitated reactor. In this embodiment, theexchanges are excellent and the precipitated metals are driven out ofthe fluidised bed whereas the larger zinc granules stagnate there untilthey reach a very small size. The drawback of this system lies in thedifficulty of implementing reliable mechanical agitation in a tubularreactor of large height. Any mechanical system placed in such conditionsis chemically attacked and abraded by the cements. To operate, thesesystems have to implement delicate embodiments such as bearings keptconstantly under pressure of a pure and neutral solution.

The document U.S. Pat. No. 5,227,138 relates to a device designed todisplace a biological liquid in a capillary tube wherein a ferromagneticpiston driven externally by a permanent magnet is made to move. Thisdevice is intended for biological uses.

The document U.S. Pat. No. 5,222,808 describes a mixture of two liquidsin a capillary tube. It makes use of a magnetic agitation system usingone or more magnetic cores moved by a variable external magnetic field.The magnetic cores are formed by microscopic powders which are directedin the field lines forming aggregates. This device is also intended forbiological uses.

OBJECT OF THE INVENTION

The object of the present invention is to reduce the drawbacks proper tothe remote activation technologies described above. Its object is toachieve a cementation process of metals with optimum yield. Its objectis to propose an installation with a high unitary processing capacity.Its object is also to enable greater agitation of the fluid solutioninvolved in the reaction to be activated while limiting the number andcost of the activation means.

The activation process according to the invention is characterized inthat:

solid ferromagnetic particles fluidised in the current of the solutionare used, being the seat of a deposition when the cementation reactiontakes place, the particles having a predetermined granulometry,

the solution is injected via the bottom of the reactor causing anascending flow of the solution in a vertical direction parallel to thewalls, whereas the solid ferromagnetic particles are introduced at thetop part of the fluidised bed.

A The impacts caused between the solid ferromagnetic particles and thereactor walls enable the metallic deposits to be detached continuously.

Preferably, the agitating means is moved in an alternating movementbetween a first extreme position and a second extreme position situatedin such a way that the agitation zone is able to appreciably span thewhole of said portion of space. The movement of the agitating means canbe limited to a to-and-fro translation movement. The processing capacityis high, as there is no limitation of the cross-section of the reactor,which can have the required width or diameter to treat a given flow rateof solutions while preserving a limited air-gap. The agitating meanscomprise a plurality of electromagnets sequentially supplied withperiodic currents to create an electromagnetic field able to direct theferromagnetic particles alternately in two distinct directions. The thinlayer of fluid in the active zone enables a maximum effect of themagnetic forces to be obtained while also having a high flow rate whichis not possible in cylindrical embodiments of the state of the art.

Alternatively or cumulatively, other agitating means can be provided,for example at least one ultrasonic transducer, at least one of saidwalls being lined with a flexible membrane containing a gel able totransmit the ultrasounds, said transducer having a head in contact withsaid flexible membrane.

According to another feature of the invention, the object of the latteris also to achieve a device for implementation of the process describedabove and comprising means for introducing the solution via the bottomof the reactor causing an ascending flow of the solution in a verticaldirection parallel to the walls, whereas the solid ferromagneticparticles are introduced at the top part of the fluidised bed.

According to an alternative embodiment, the device comprises means forinjecting reactive metal wires able to be used for injecting liquidchemical adjuvants.

Preferably, both of the walls are shaped in such a way that theirexternal surface is geometrically defined by a set of segments ofstraight lines parallel to one and the same geometric axis and bearingon any curve extending in a plane perpendicular to said axis, thedistance between each segment of one of the walls and the other wallbeing constant. This geometrical definition covers in particular thecase where both the walls are flat or cylindrical with a circular base.

Preferably, the device comprises in addition drive means for driving thefluid in a driving direction parallel to said geometric axis, the wallscomprising, on their faces facing one another, asperities formingrestrictions designed to cause local accelerations of the fluid.

The invention is mainly applicable to cementation of non-ferrous metals,both in the primary metallurgy sector and in that of decontamination ofground surfaces and of solutions charged with heavy metals.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become more clearlyapparent from the following description of different embodiments of theinvention given as non-restrictive examples only and represented in theaccompanying drawings in which:

FIG. 1 schematically represents a device according to a first embodimentof the invention, in cross-section along a vertical plane;

FIG. 2 represents a cross-section along the plane II—II of FIG. 1;

FIG. 3 represents a side view of the device of FIG. 1;

FIG. 4 represents a cross-section of a device according to a secondembodiment of the invention;

FIG. 5 represents a cross-section of a device according to a thirdembodiment of the invention;

FIG. 6 represents a cross-section along the plane VI—VI of FIG. 5;

FIG. 7 represents a cross-section of a device according to a fourthembodiment of the invention;

FIG. 8 represents a cross-section of a device according to a fifthembodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIGS. 1 to 3, an activation installation 10 of acementation reaction comprises a reactor 12 of rectangular cross-sectionin a plane perpendicular to that of FIG. 1, forming a recipient, withtwo flat large walls 14, 16 facing one another and two walls of smalldimensions 18, 20. Each of the large walls measures 2 meters in heightand 16 centimeters in width. The distance between the two large walls isabout 4 cm.

The cover forming the top face of the reactor is equipped with a loadinghopper 22 designed to feed the reactor with iron balls 24 constitutingthe reactive metal in this instance. The bottom face is equipped with afeed pipe 26 located downstream from a pump 27 and equipped withinjection nozzles 28. The solution charged with the noble metal to beextracted is able to be introduced by means of this pipe. The reactorcan be drained by means of a valve 30. An outlet pipe 31 is located inthe upper part of the reactor. Flow of the mixture in the recipientformed by the reactor is therefore ascending and globally follows avertical direction 100 parallel to the flat walls 14, 16.

Two fixing flanges 32, 33 are arranged on the cover of the reactor, eachsupporting a side rail 34, 36. These rails guide in translation acarriage 38 formed by two longitudinal girders 40, 42 and twocross-members 44, 46, surrounding the reactor 12, the cross-memberscomprising linear bearings operating in conjunction with the rails 34,36. The carriage 38 supports in the example four pairs of electromagnetsA, A′, B, B′, C, C′ and D, D′, the poles whereof are connected to oneanother by cores made of laminated silicon steel plate.

The installation also comprises a driving device 50 of the carriage,constituted by an electric motor 52 equipped with a speed variatorcoupled to a reducing gear train 54 the output shaft 56 whereof drivestwo coaxial cylinders 58, 60 acting as winches. A cable 62, 64 linkseach of the winches 58, 60 to one of the longitudinal girders 40, 42.

The device operates in the following manner:

The electromagnets are excited periodically pair by pair for example(AA′) then (BB′) then (CC′) then (DD′) then (DC′) then (CB′) then (BA′)so as to cross the field lines and force the ferromagnetic particles tobe directed alternately in the two directions xx′ and zz′ as indicatedin FIG. 2. Excitation of the electromagnets is controlled by means of aprogrammable controller in the previously indicated order.

At the same time, the carriage 38 supporting the electromagnets movesslowly downwards and upwards in a direction parallel to the axis 100.The periodicity of the electromagnet support carriage is such that itperforms one back-and-forth movement in a time comprised between 10seconds and 2 minutes.

By co-ordinating the movement of the carriage 38 and the excitation ofthe electromagnets, it is possible to program the agitation to make itmaximum in the most critical zones such as those situated at the levelof the solution injectors 28. By optimum adjustment of the excitationtimes and the speed of the electromagnet support carriage 38, it is alsopossible to partially force the particles situated at the bottom of thefluidised bed to move up towards the top. This presents an advantage asthe upper part of a fluidised bed is always a zone of small activity andhigh porosity (few particles present and small particle size).

To illustrate the effect of electromagnetic activation, cementationtests of a diluted copper solution were carried out with the reactor ofFIGS. 1 to 3. The solution initially contained 2.5 g/l of copper insulphate form in solution in sulphuric acid at pH 1.5.

A first test was carried out in a simple fluidised bed without anymagnetic field action. The reactor had previously been loaded with 25 kgof iron balls with a diameter of 3 mm. The solution, the initial volumewhereof was 1 m³, was injected at a rate of 2.5 m³/hour. After 25minutes, an account of the passage of the solution in the fluidised bedwas made. The copper cements were decanted, washed and filtered on aBuchner filter then dried and weighed. The content of the analysedsolution after the first run was 0.43 g/l. The quantity of cementweighed was 1.72 kg, the rest of the copper having remained fixed on theiron balls contained in the bed. The pH of the solution increased from1.5 to 1.97. The iron content of the copper after remelting was analysedand was found to be equal to 2%.

The iron granules contained in the bed were washed with water for 1 hourunder magnetic activation so as to remove the residual copper. 0.260 kgof copper was collected which represented a copper yield of 95.6%. Theiron content of the solution was analysed and found to be equal to 2.36g/l which enabled the value of the iron yield to be established at 74%.

The same test was repeated under strictly identical conditions but withthe electromagnets excited according to the following cycle: excitationfor 30 μs in the order AA′, BB′, CC′, DD′ followed by a 15 μs pause thenexcitation again for 30 μs in the order DC′, CB′, BA′ followed byanother 15ps pause before restarting the cycle. At the same time, thecarriage was moved at a constant speed of 10 cm/s in a to-and-fromovement from one end of the reactor to the other.

This time 2.205 kg of copper were recovered containing 1.1% of iron inthe form of cements, i.e. an extraction yield of about 98.6%. The ironcontent measured in the solution was then 1.95 g/l and the final pH wasequal to 1.53. The iron yield calculated from the analyses of thesolution was then 96% against 74% in the previous test.

According to a second embodiment of the invention, illustrated by FIG.4, the reactor 12 is modified so as to provide the two large walls 14,16 with asperities, for example, one or more restrictions formingventuris and designed to cause a local acceleration of the fluid. Inpractice, two venturis 70, 72 are positioned 60 cm and 120 cm from thebottom of the reactor. The venturis are formed by polypropylene stripsfolded and welded as indicated in FIG. 4. The slit of the venturis is1.5 cm wide and their height is 20 cm.

To illustrate the influence of the venturis, two series of tests werecarried out. In these sections of the reactor, the velocity wasincreased from 11 cm/s to about 30 cm/s over 10 cm, then dropped backfrom 30 to 11 cm/s over the 10 cm above. To increase the turbulence inthe venturis a certain proportion of large particles were placed in thetwo upper compartments, i.e. 0.8 kg of large shot with a granulometry ofabout 4.5 mm in each of the compartments, which represented a proportionof the charge of about 6%.

The first test was carried out without activation of the electromagnets.2.12 kg of copper cement containing 1.3% of iron were recovered afterpassing 1 m³ of solution, i.e. a quantity of copper recovered of 2.09kg. The copper content of the solution was measured at 0.345 g/l. Thequantity of cements remaining on the iron was evaluated by weighing at0.14 kg. The final iron content of the solution was 2.06 g/l and the pHwas 1.63. The iron yield was therefore about 89.5%;

The same experiment was repeated but with the electromagnets activated.2.25 kg of copper cement containing 1.12% of iron were then recovered,which corresponds to a recovery yield of 98.5%. The iron content of thesolution was measured at 1.985 g/l, i.e. a yield of about 97.5%. The pHof the solution hardly varied as it went from 1.5 to 1.52.

According to a third embodiment of the invention, illustrated by FIGS. 5and 6, the reactor 112 has a crown-shaped cross-section. The solution tobe treated is contained between an inside wall 114 and an outside wall116, both cylindrical. The distance between the two walls, whichdetermines the thickness of the solution layer, is about the same as inthe previous example, i.e. 4 cm. Two carriages 118, 120 are thenrequired to support the electromagnets. Movement of the two carriages iscoordinated by a common geared motor 122 driving three coaxial winches124, 126, 128. The cable of the inner carriage comprises four slings.

According to a fourth embodiment of the invention, illustrated by FIG.7, the apparatus enables indirect cementation to be implemented by usinga much more reactive metal than iron, such as zinc or aluminium forexample. To do this, the reactive metal (zinc or aluminium) isintroduced continuously into the reactor in the form of wires which areunwound in guides 132, 134, 136 opening out into the reactor 12. Theguides are formed by polymer (polypropylene, polyethylene . . . ) tubes.Two or three guides are placed per meter of width. The wires then invadethe reaction zone in which they form skeins which increase theturbulence and offer a large contact surface with the iron balls whichtake a potential close to that of the wire. Under these conditions, themetals can precipitate over the whole surface of the iron balls indirect or indirect contact with the wires. The iron balls than only actas mechanical agitator whereas the wire is consumed and is regularlyreplaced by unwinding thereof in the reactor.

According to a fifth embodiment of the invention, representedschematically in FIG. 8, the moving carriage 150 is equipped withultrasonic transducers 152, 154 designed to bring about activation byultrasounds. Application of ultrasounds is possible due to the verydesign of the reactor in a thin strip. In this case, to enable theultrasonic waves to propagate through the fluidised bed, the transducerheads have to be in permanent contact with the medium. To do this, adouble shell is placed on the reactor 155, which is therefore lined withtwo flexible membranes 156, 158 containing a gel 160, such as those usedfor performing echographies or a colloidal silicone gel or any otherform of gel.

From the economics standpoint it is more costly to achieve an ultrasonicactivator than an electromagnetic activator, but it may be worthwhilewhenever any presence of iron or ferromagnetic material is to beavoided.

Naturally the invention is not limited to the examples of embodimentspresented above. In particular, the recipient acting as reactor can takeany form enabling two walls of large height and constant cross-sectionto be defined by a plane perpendicular to the direction of translationof the carriage, at a small distance from one another. Thus, if thedirection of linear movement of the carriage is chosen as referenceaxis, it appears that each of the large walls of the reactor must be ashell, in the geometrical sense of the word, the envelope whereof is acylindrical portion of surface whose generating lines extend in alongitudinal direction. What is meant here by cylindrical portion ofsurface is a surface formed by a set of segments of straight linesparallel to the reference axis and bearing on any curve forming its baseline. The base line curve can itself be a segment of a straight line, asillustrated by the reactor of FIGS. 1 to 3, or a circle, as illustratedby the reactor of FIGS. 5 and 6.

Furthermore, the large walls can be of any dimensions. For an industrialuse for example, walls 4 meters in width and 4 meters in height,arranged at a distance of 10 cm from one another, enable a flow rate of190 m/hour, and therefore of 5,000 tons/year for a solution with 3 g/lof copper, to be obtained with a mean linear velocity of the fluid ofabout 12 cm/sec. Satisfactory activation is then obtained with 80 pairsof electromagnets. If it was chosen to manufacture cylindrical crownreactors, the dimensions would be comprised between 1.3 and 2.6 metersin diameter.

What is claimed is:
 1. A process for activating a physical and/orchemical reaction in a mixture subjected to agitation and comprising asolution charged with a noble metal in ionic form and a reactive metalintroduced in solid form and freely dispersed in the solution so as toobtain a cementation from the reactive metal reacting to the noble metalpresent in the solution, comprising: injecting the solution via thebottom of a reactor having two walls facing one or another and close toone another, the mixture filling the space between the two walls andforming therein a layer of small thickness and of great length in adirection defined by a geometric axis parallel to the walls, anagitating means movably attached to the reactor including a plurality ofelectromagnets, the electromagnets being arranged outside the reactor toact through said walls on an agitation zone covering a part of saidlayer and having a small dimension in the direction of the geometricaxis; causing an ascending flow of the solution in a vertical directionparallel to the walls so as to form a fluidised bed; introducing solidferromagnetic particles, at the top part of the fluidised bed, whichparticles are the seat of a disposition when the cementation reactiontakes place, agitating the mixture by activating the electromagnetssequentially supplied with periodic currents to create anelectromagnetic field able to direct the ferromagnetic particlesalternately in two distinct directions; moving the agitating means in analternating movement between a first extreme position and a secondextreme position so that the agitation zone spans appreciably the wholeof said space located between the two walls; and cementing the noblemetal from the mixture.
 2. A device for implementing the processaccording to claim 1, for activating a physical and/or chemical reactionin a mixture subjected to agitation and comprising a solution chargedwith a noble metal in ionic form and a reactive metal introduced insolid form and freely dispersed in the solution so as to obtain acementation consisting in replacing the noble metal present in thesolution by said reactive metal, a device comprising: a reactor designedto contain the mixture to be agitated, the reactor having two wallsfacing one another and close to one another, the mixture filling thespace between the two walls and forming therein a layer of smallthickness and of great length in a direction defined by a geometric axisparallel to the walls, causing an ascending flow of the solution in avertical direction parallel to the walls so as to form a fluidised bed,means for injecting the solution via the bottom of the reactor, meansfor causing an ascending flow of the solution in a vertical directionparallel to the walls, means for introducing solid ferromagneticparticles at the top part of the fluidised bed, which particles are theseat of a deposition when the cementation reaction takes place,agitating means comprising a plurality of electromagnets sequentiallysupplied with periodic currents to create an electromagnetic field ableto direct the ferro-magnetic particles alternately in two distinctdirections, the electromagnets being arranged outside the reactor to actthrough said walls on an agitation zone covering a part of said layerand having a small dimension in the direction of the geometric axis, andmeans for moving the electromagnets in an alternating movement between afirst extreme position and a second extreme position so that theagitation zone spans appreciably the whole of said space located betweenthe two walls.
 3. The device according to claim 2, comprising means forintroducing the reactive metal in the form of metallic wires.
 4. Thedevice according to claim 3, wherein the means for introducing thereactive metal comprise guides opening out into the reactor.
 5. Thedevice according to claim 4, wherein the guides are formed by polymertubes.
 6. The device according to claim 2, wherein both of the walls areshaped in such a way that their external surface is definedgeometrically by a set of segments of straight lines parallel to one andthe same geometric axis and bearing on any curve extending in a planeperpendicular to said axis, the distance between each segment of one ofthe walls and the other wall being constant.
 7. The device according toclaim 6, wherein both of the walls are flat or cylindrical with acircular base.
 8. The device according to claim 2, wherein the wallscomprise asperities forming restrictions, inside the space bounded bythe walls, designed to cause local accelerations of the solution.
 9. Thedevice according to claim 2, comprising a support carriage of theelectro-magnets, the means for moving the electromagnets comprisingmeans for driving the carriage.